Well, if Lawson were to do it, things would work out because he likes
Nabiki.

Oh, I don't know about that.  I've read enough of Lawson's work
to believe that he has an honest inner critic.  You saw how the
first bit went - Nabiki turned out a lot angrier and more volitile
than he said he expected.  If he were to push through with writing
the story, it would probably go in different directions than he
initially planned.  This is just guesswork, of course, but I think
a lot of writers find that stories take on a life of their own once
you start them.  I know mine do.

Now this is very true...  "Hey Kasumi, a little more to the left?  Thanks..."  ;)

In reality, of course, Nabiki would simply get stomped on:

Reality?  Heh.  Good one.

Where?  If I wanted reality, I'd get a life, natch.

Ranma:  Would die before he'd recognize her authority over him. He would
fight her like a boy possessed; Nabiki would have to escalate things
dramatically just to win a temporary victory.

I think so too.  He wouldn't fight her physically, but they'd clash
in all sorts of ways.  I think that could be the central source of humor
and conflict in a story like this one.

This is the main flow in any story involving Ranma, and why Takahashi kept piling stuff
on his back... cause the guy just won't stay down!  This, more then anything else in my
opinion is what defines Ranma as a character, he won't give up! (Though he does like a
challenge.)

Kunou:  She may be able to get him to see the truth about the
'pig-tailed girl', but nothing short of Akane directly telling him that
she doesn't love him would have the least effect.

Even that probably wouldn't help.  I think, more likely, Nabiki might
decide to try to deflect Kuno's affections onto someone else in order
to help her sister (perhaps Ranma, perhaps not).  Then, of course,
she would learn (to her horror) about the "I must have them both!"
principle.  That's the sort of thing I'd picture as going wrong when Nabiki
tries to solve things.  It could be funny.  There are other possibilities,
of course, but I think it would be a bit disappointing if Nabiki, for some
reason, could just talk Kuno out of his obsessions and fantasies.

Better yet, as head of the household, what if Kuno were to pursue Nabiki for once, and
actually be trying to get at Akane and the pig tailed girl?  OH now that would get
Nab's goat!

Kodachi:  Nabiki would very likely wind up in a lot of pain, if not
dead. She simply can't hope to fight Kodachi in any meaningful way.

I don't know.  I'm not sure where Nabiki would actually come into conflict
with Kodachi.  It would depend on how events went in the story and
where Kodachi got involved.  If Nabiki is clashing with Ranma, she might
not have any reason to cross Kodachi's path directly.

Shampoo:  This amazon would kill her in an instant if she felt Nabiki
posed a serious threat to her mission.

Have to disagree with you here, and this is just my take on the way the
Amazons (Shampoo and Mousse, anyway) actually work.  They talk
the talk about killing obstacles and such, but they never seem to get
around to walking the walk, for whatever reason.  I just don't think they're
killers at heart, in spite of the way they like to talk about it.  They have
a lot of opportunities in the original story to follow through with a kill,
but they always avoid it one way or another.  Anyway, I think you
could have a perfectly valid and in character story in which Shampoo
doesn't want to stoop to physically threatening a non-fighter like Nabiki.

Ukyou: Least likely to physically harm Nabiki, she is far to stubborn,
canny, and strong-willed to let anyone influence her goal of marrying
Ran-chan.

Again, it kind of depends on whether Nabiki ever thinks Ranma and
Akane could be engaged at all.  And it would be kind of funny to seeNabiki trying to
deflect Ukyou's attentions away from Ranma and failing.

I always saw Ukyo as the craftiest of Ranma's fiancee's, a direct confrontation between
her and Nabiki would be entertaining... Till Nabiki got the upper hand, that is.>

Ryouga:  The fact Nabiki is a girl would probably be enough to save her

>from serious injury. Probably. She would be in a position to cause him

an unbearable loss of face, so all bets are off on her chances of
survival.

I can't say I agree on this one either.  Ryouga is dead-set against the
idea of picking on the weak.  In a battle of wits, he's got a serious disadvantage,
and given his heart-on-his-sleeve nature, I think Nabiki could learn to push
his buttons pretty handily.  I could see a story in which Ryouga becomes
Nabiki's unwilling catspaw for a lot of different schemes or ideas, given his
credulous nature.

This has been done at least once or twice, and very IC in my opinion

The point of all this is that if Nabiki tap-danced with giants, she
would likely be stepped on.

That's one interpretation, though I think there are other valid possibilities.
Nabiki's a pretty good manipulator, and she could probably balance the
scales with brains against brawn.  I think the "New Roles" Nabiki would
get more emotionally involved than the original, and thus she'd leave herself
open more, but it could be written plausibly without her ending up in the
hospital.  Or the morgue.  Or whatever.

I could even see a version in which she and Ranma end up being the ones
who fall for each other.  After all, Ranma and Akane thrive on their mutual
antagonism.  If Nabiki ended up respecting Ranma as an adversary, who
knows where it might lead?

This is an interesting idea, (and has been done fairly well before), but I always
thought that part of what draws Ranma to Akane is her kindness, (Even if not to him
that often) as well as her competitive nature.